Local impact of solar variation on NO₂ in the lower mesosphere and upper stratosphere from 2007 to 2012

Abstract. MIPAS/ENVISAT data of nighttime NO₂ volume mixing ratios (VMR) from 2007 until 2012 between 40 km and 62 km altitude are compared with the geomagnetic Ap index and solar Lyman-α radiation. The local impact of variations in geomagnetic activity and solar radiation on the VMR of NO₂ in the lower mesosphere and upper stratosphere in the Northern Hemisphere is investigated by means of superposed epoch analysis. Observations in the Northern Hemisphere show a clear 27-day period of the NO₂ VMR. This is positively correlated with the geomagnetic Ap index at 60–70° N geomagnetic latitude but also partially correlated with the solar Lyman-α radiation. However, the dependency of NO₂ VMR on geomagnetic activity can be distinguished from the impact of solar radiation. This indicates a direct response of NO_x (NO + NO₂) to geomagnetic activity, probably due to precipitating particles. The response is detected in the range between 46 km and 52 km altitude. The NO₂ VMR epoch maxima due to geomagnetic activity is altitude-dependent and can reach up to 0.4 ppb, leading to mean production rates of 0.029 ppb (Ap d)⁻¹. Observations in the Southern Hemisphere do not have the same significance due to a worse sampling of geomagnetic storm occurances. Variabilities due to solar variation occur at the same altitudes at 60–70° S geomagnetic latitude but cannot be analyzed as in the Northern Hemisphere. This is the first study showing the direct impact of electron precipitation on NO_x at those altitudes in the spring/summer/autumn hemisphere.